Laminar Burning Velocity Measurement Using the Filtered Broadband Natural Emissions of Species

Abstract

The laminar flame speed plays a fundamental role in understanding hydrocarbon chemistry and is the basis of turbulent flame modeling. The main objective of this research is to visualize a flame and measure its speed using a new optical diagnostic technique, which is based on the natural radiative emissions of various species, mainly hydrocarbons, water, and carbon dioxide. This new technique has two main advantages over the Schlieren method. First, only one optical access is needed, and second, the preheat zone and the flame front can be observed, and their radiative emission can be quantified. In addition, the condition of the unburned gas in front of the flame can be studied using the new technique to evaluate some of the hypotheses behind unstretched flame speed measurement, such as the trivial change of the unburned gas state. To evaluate and validate the new optical diagnostic technique, the flame propagation of methane, oxygen, and nitrogen mixtures was studied using species infrared radiative emissions at broadband wavelengths in the range of 3.239–3.629 μm. The flame propagation was pictured using a high-speed infrared detector under constant pressure condition. The study was conducted at a mean initial gas pressure of 1.005 ± 0.0005 bar, a mean initial gas temperature of 296 ± 1.1 K, and four different equivalence ratios of approximately 0.8, 0.9, 1.0, and 1.1. The flame speed measured using the new technique aligns well with the measured data from the literature using the Schlieren technique and simulated data using a detailed mechanism at the studied conditions. The measured data also show the presence of the radiative emission from the unburned gas. This observation contradicts the assumption of trivial temperature change of the unburned gas mixture during the flame propagation, which is widely used for unstretched flame speed measurement relative to the unburned gas state.

Methane Laminar Flame Speed Measurement at High Gas Temperature Using Rapid Compression Machine-Flame (RCM-Flame)

“…The effect of the density threshold does not play a significant role in the simulation of the flame front location. 3 The simulations were performed at a gas temperature and pressure of 626 K and 5 bar, respectively, with a mixture composition of CH 4 : 0.0741, O 2 : 0.1484, and N 2 : 0.7778 (partial pressure). The pressure−time histories in the three chambers are shown in Figure 7.…”

“…The flame propagation was modeled at three-volume sizes as 0.07 (the same as RCM chamber size), 0.55, and 4.37 L. A density threshold was used to determine the flame front location in the simulation framework with a mean value of 1.10 ± 0.5% kg/m 3 . The effect of the density threshold does not play a significant role in the simulation of the flame front location . The simulations were performed at a gas temperature and pressure of 626 K and 5 bar, respectively, with a mixture composition of CH 4 : 0.0741, O 2 : 0.1484, and N 2 : 0.7778 (partial pressure).…”

“…Therefore, the upper postprocessing limit was selected when the gas pressure reached ∼1% of the initial gas pressure, based on previous work. 3 The radius of the flame, R j (subscript j represents X-and Yaxis), was extracted from the image data set recorded using the high-speed camera. These recorded images were individually postprocessed using an in-house code developed using the Image Processing Toolkit within MATLAB.…”

“…Thus, the gas pressure inside the chamber rose, which affected the flame propagation process. Therefore, the upper postprocessing limit was selected when the gas pressure reached ∼1% of the initial gas pressure, based on previous work …”

“…Laminar flame speed is one of the critical characteristics of a hydrocarbon combustion process. Researchers have measured laminar flame speeds using devices such as the burner, diverging channel, constant volume chamber, and shock tube . The following is a brief review of recent works relevant to the current research.…”